Has it been done? Is it possible?
I'd like to see/create a pneumatic tube-like transport system in a HL map. I'm sure it's possible with trigger pushes. You could select your destination from a row of func_buttons, each would trigger a different door to slide down somewhere in the tube, so you can choose where you stop.
Im pretty sure I saw something like this in a Garry's Mod RP map once.
(If you've seen Futurama you'll know what I mean)